- C Mary Schooling, professor
- 1City University of New York School of Public Health and Hunter College, New York, NY 10035, USA
Clinical guidelines are increasingly based on experimental evidence from trials. Few randomized controlled trials give the effect of diet or nutrients on major health outcomes. Dietary guidelines or recommendations tend to be based on observations from prospective cohort studies, possibly describing the eating habits of people who happen to live longer for a multitude of reasons rather than the effects of a particular diet or nutrient.
Dietary guidelines may also be designed to provide recommended intakes of specific nutrients, sometimes based on the lowest level of evidence (that is, opinion) and reflecting the assumption that normal intakes in Europe or North America represent those that are optimal.1 As such, it is hardly surprising that dietary guidelines are not always confirmed by experimental evidence from trials, such as the harmful effects of saturated fats2 or the benefits of calcium.3 In a linked paper (doi:10.1136/bmj.g6015),4 Michaëlsson and colleagues question the role of milk, an item that often features in dietary guidelines.5 Based on observational evidence, Michaëlsson and colleagues raise the possibility that milk could increase the risk …